1. Field of the Invention
This invention relates to semiconductor devices, and more specifically to techniques for verifying the microstructure of lead-free interconnects in semiconductor assemblies.
2. Background of the Invention
Because of the environmental effects of lead (Pb), transitioning to lead-free soldering is currently on the agenda of many companies that manufacture electronics equipment. Lead is a primary constituent in conventional solder, which is used in all types of electronics. The disposal of such electronics has raised concerns about the amount of lead that is entering the environment through landfills or other avenues. To minimize the effect on the environment, legislation has been enacted in various parts of the world to mandate or encourage the transition to lead-free soldering.
Currently, the electronics industry is experimenting with lead-free, tin-based solders (e.g., tin-based alloys containing elements such as silver, copper, nickel, bismuth, gold, or the like) to provide means for interconnecting flip chips and other semiconductor devices with external circuitry. Unfortunately, lead-free, tin-based solders are typically substantially more rigid than their lead-based counterparts, making chip circuitry more susceptible to cracking and delamination. As a result, electronics manufacturers are working to develop processes and techniques to mitigate and/or compensate for the inherent rigidity of lead-free, tin-based solders.
The microstructure of lead-free, tin-based interconnects falls into one of two categories: (1) undercooled solidification type; and (2) controlled solidification type. The “naturally” solidified tin-based interconnect generally falls into the first category (i.e., the undercooled category), at least partly because the heterogeneous nucleation of beta-tin is difficult. This type of solidification produces a rigid, high-stress interconnect which is prone to chip circuitry fracture and/or delamination. In certain cases, interconnect size contributes to the undercooling state.
Interconnects of the controlled solidification type are softer and more ductile, and thus more reliable, than their undercooled counterparts. Such interconnects typically contain larger grains beta-tin, formed during a controlled heating and cooling process. These grains are surrounded by intermetallics or intermetallic compounds. In addition to carefully controlling the heating and cooling process, research has produced techniques to reduce undercooled solidification by adding different alloy elements to the tin-based solder.
Nevertheless, even if interconnects of the controlled solidification type can be produced, verifying the microstructure can be difficult, particularly in flip-chip applications where the interconnects are substantially hidden from view. A cross-section is required to determine if the interconnects have achieved a desired microstructure. However, creating such cross-sections is time-consuming and may require specialized human and tooling resources. Thus, such techniques may not be suitable for verifying the integrity of interconnects on a full-scale production line.
In view of the foregoing, techniques are needed to quickly identify whether a desired microstructure (e.g., controlled solidification type) has been achieved in flip-chip interconnects or other interconnects that utilize lead-free solder. Ideally, such techniques could be implemented on a full-scale production line to provide substantially instantaneous quality control feedback.